The present invention relates to a pressed retainer and a ball bearing comprising the pressed retainer, more particularly to a pressed retainer and a ball bearing comprising the pressed retainer structured so as to be unlikely to seize even when used in areas where lubrication conditions are strict, such as the crankshaft of a two-cycle engine.
A pressed retainer for a conventional ball bearing is produced as described below. As shown in FIG. 6 for example, two ring-shaped retainer plates 52, 52 provided with ball retaining portions 53, 53, . . . disposed at equal intervals in the circumferential direction are faced to each other so that two mating ball holding portions, that is, one ball holding portion 53 and the other ball holding portion 53 form a pocket 56, and flat portions (joint portions) 54, 54 are joined and secured with rivets 7. A corrugated retainer 51 for this ball bearing, with balls disposed in the pockets 56, is disposed in a ring-shaped space between the inner and outer races (not shown) of the ball bearing.
FIG. 7A is a sectional plan view showing the pressed retainer 51 for the above-mentioned conventional ball bearing, taken in the circumferential direction of the pitch circle of the bearing. In this kind of ball bearing, when its race (inner race or outer race) rotates relatively, both the balls 5 and the retainer 51 rotate simultaneously(around the race). However, the ball 5 rotates around the race and also rotates on its axis while making contact with the forward surface in the rotation direction (advancing direction) and the inner wall surfaces of the pocket 56. In other words, when the pressed retainer 51 rotates around the race, the ball 5 makes contact with the inner wall surfaces 56a, 56b of the pocket 56 of the pressed retainer 51 in the advancing direction while rotating on its axis Z. The shapes of the contact areas of the inner wall surfaces 56a, 56b are shown in FIG. 7B as viewed fromarrowed line A--A of FIG. 7A.
Recently, ball bearings comprising ceramic balls being usable in strict environmental conditions (temperature is high, high strength is required, lubrication is insufficient, and the like), have begun to be used. Such ceramic balls have begun to be used in some areas wherein the use of ordinary bearings is difficult, for example, in areas wherein heat resistance, strength, high speed resistance, insulation, etc. are required, because the ceramic balls are superior in light weight, heat resistance, high hardness, low coefficient of linear expansion, corrosion resistance, high hardness, etc.
In the conventional ball bearing comprising the retainer 51 shown in FIG. 6, when the ball 5 makes contact with the inner wall surfaces of the pocket 56 and rotates on its axis Z as shown in FIG. 8, and assuming that an angular speed during the rotation on its axis Z is .omega., and a radius from a given surface of the ball 5 to the axis Z (vertical distance) is r, the circumferential speed v of the surface of the ball 5 is represented by .omega..multidot.r . Therefore, the ball 5 makes contact with the inner wall surfaces of the pocket 56 of the pressed retainer 51 at positions where the rotation radius of the ball 5 becomes maximum, that is, areas of positions P, P where the circumferential rotation speed becomes maximum. Accordingly, the ball bearing comprising the conventional pressed retainer 51 is used in a condition wherein seizure is likely to occur, whereby the ball bearing causes the problem of relatively short service life.
In addition, if ceramic balls, having no lubrication capability in themselves and intended to be used in strict environmental conditions, are used in improperly lubricated areas, the balls having high hardness may cut away the track surfaces of the mating inner and outer races made of steel, or may significantly wear the retainer made by pressing. To solve these problems, an attempt has been made to use a super engineering plastic material having lubrication capability for the retainers of turbocharger bearings used at high temperature and high speed. However, since the material is a plastic material, it is subjected to limitations and restrictions with respect to heat resistance and wear resistance.